1. Field of the Invention
The present invention relates to an optical multiplexing method and an optical multiplexer, for multiplexing lights of different wavelengths which are utilized in various optical communication systems, and an optical amplifier using the same. In particular, the present invention relates to a technique for realizing the multiplexing of lights with a simple optical circuit configuration.
2. Related Art
An optical amplifier is one of key components realizing the long distance and large capacity of optical communication system. Types of optical amplifier are classified into a laser amplifier using stimulated emission from a population inversion medium, and an amplifier based on a nonlinear optical effect such as Raman scattering or Brillouin scattering. As the laser amplifier, there are a rare earth element doped fiber amplifier and a semiconductor laser amplifier using a semiconductor amplification medium, the former being operated with the photo excitation and the latter being operated with the inlet current excitation. Among these optical amplifiers, the rare earth element doped fiber amplifier has a high advantage in performance, for example, bit rate free, high gain, low noise, broad band, low coupling loss, low polarization dependence, high efficiency and the like. Among the rare earth element doped fiber amplifiers, an erbium (Er) doped fiber amplifier (to be referred to as EDFA herebelow) is typical, and is now in practical use in optical fiber communication systems. The performance (gain, NF) and cost of such an optical amplifier for optical communication represented by EDFA and the like are key factors of optical communication systems, and it is important how the reduction of cost is achieved while maintaining the required performance.
The cost required for a pumping light source and a multiplexing system thereof constitutes a large proportion of the cost of optical amplifier for optical communication. As one of means for achieving the cost reduction of pumping light source, there has been known a technique in which a low cost semiconductor laser (LD) for commercial-off-the-shelf (for example, DVD or CD-R) is adopted (refer to Japanese Unexamined Patent Publication No. 6-318750).
However, in order to make it possible to apply an optical amplifier using a pumping light source which is at low cost although having less reliability, as the optical amplifier for optical communication in which the strict reliability (duration of life) is required, it becomes important to improve the reliability up to a predetermined value while keeping a cost merit.
As one method for improving the reliability of pumping light source utilizing low cost LD, it is effective to adopt a redundant configuration improving the reliability up to a predetermined level by applying multiple pumping LDs, although such an LD has less reliability when used in a simple body. In the redundant configuration of known optical amplifier, even if the unit cost of pumping LD is low, the cost required for optical components of multiplexing system corresponding to the redundant configuration is high. Therefore, it is difficult to expect the cost-reduction of the whole optical amplifier. Specifically, with an increase of the number of pumping LDs, the cost is increased due to the addition of high-cost optical multiplexer (including an increase of assembly cost with the increase of the number of components), and also required output power of single pumping LD is increased due to an increase of insertion loss. Therefore, it is desirable that the number of components of multiplexing system can be reduced to realize the low cost.
As conventional techniques for multiplexing lights of different wavelengths, there has been proposed a technique multiplexing light utilizing a tilted grating (refer to Japanese Unexamined Patent Publication No. 8-171031 and Japanese National Patent Publication No. 2001-516468). The tilted grating is formed such that a direction of grating (a direction perpendicular to a plane where the rise of refractive index occurs) is tilted to an axial direction of optical path of an optical fiber, optical waveguide or the like, and is sometimes called a slanted grating.
Specifically, Japanese Unexamined Patent Publication No. 8-171031 discloses an optical apparatus in which a grating coupler is disposed in a waveguide arranged on a silicon substrate on which a photodiode is formed, a semiconductor laser and an optical fiber are disposed on positions opposite to the grating coupler, a transmission light emitted from the semiconductor laser is incident on the grating coupler at an incident angle of 45° and totally reflected by the grating coupler, to be sent to the optical fiber, and also a reception light of a wavelength different to that of the transmission light is output from the optical fiber to be incident on the grating coupler at an incident angle of 45°, and excited in the waveguide by the grating coupler, to be received by the photodiode. Further, Japanese National Patent Publication No. 2001-516468 discloses an optical apparatus in which a lens is formed on a cladding surface of an optical fiber, on which is written with titled fiber Bragg grating (FBG), to be coupled with the FBG, and a light guided in the optical fiber via the lens is reflected by the tilted FBG, to be guided into the core.
The optical multiplexing method using the tilted grating as described above is considered to be one of techniques effective for reducing the number of components of pumping light multiplexing system in the optical amplifier, since lights of a plurality of wavelengths can be multiplexed with a simple configuration, compared with the case where a plurality of pumping lights is multiplexed sequentially in a three using WDM couplers, polarization combining devices and the like in a pumping system of the optical amplifier (for example, refer to FIG. 18).
However, in the case where the conventional optical multiplexing technique using the tilted grating described above is applied to the pumping light multiplexing system of the optical amplifier, there is caused the following problem. Namely, when the optical multiplexing technique disclosed in Japanese Unexamined Patent Publication No. 8-171031 is applied to the optical amplifier, the pumping lights are multiplexed by means of a bulk type grating formed on the waveguide. The amplification medium used in the optical amplifier such as EDFA and the like is typically of a fiber type, and a combination thereof with the bulk type grating has a possibility of an increase of insertion loss to increase required output power of a simple pumping LD. Therefore, such a combination is not necessarily effective for the cost reduction.
Further, in the case where the optical multiplexing technique disclosed in Japanese National Publication No. 2001-516468 is applied to the optical amplifier, the pumping light is introduced into the tilted grating formed in the optical fiber via the external optical fiber and the lens. Therefore, the optical circuit configuration for introducing the pumping light into such a tilted grating becomes complicated, leading a possibility of a drop of the reduction effect of the number of components by the utilization of tilted grating.
Moreover, in the above described conventional optical multiplexing technique, it is necessary to coincide high accurately the wavelengths of lights to be multiplexed with the reflective wavelength of the tilted grating, in order to obtain the high coupling efficiency. However, since the output wavelengths of the pumping light source and the reflective wavelength of the tilted grating are changed due to external factors, such as individual differences between components or a temperature variation, it becomes difficult to obtain required pumping light power. Further, in order to stably operate the optical amplifier, a function for fixing the wavelengths of the pumping lights to be multiplexed is demanded. However, the realization of such a function leads the increase of number of components and an increase of cost.